What is the isolation principle?
Make it safe before you touch it: identify, isolate, secure, prove dead and earth — every time, in that order.
The isolation principle says that no one works on an electrical circuit until it has been made safe by a fixed sequence. The circuit is identified, isolated from every source, secured with a lock and tag, proved dead by test, and earthed at the work point.
A series lighting loop carries a constant current of several amperes (commonly up to 6.6 A) at a CCR voltage that can reach kilovolts. Both are lethal, and stored charge or induced voltage can remain after switch-off. That is why a switch being off is never accepted as proof: the circuit is treated as live until it has been tested and proved dead.
- Identify and isolate. Confirm the correct circuit, switch the CCR off and put it on local control so it cannot be started remotely, then isolate the circuit from the CCR output and from every other source.
- Secure. Fit your own lock and tag so nobody can re-energise it while you work.
- Prove dead. Test with an approved indicator, checked on a known source before and after.
- Earth. Earth the circuit at the work point, on both sides of any point where it will be opened, as the site procedure requires, then start work under the permit.
Frequently Asked Questions
Q1What is the isolation principle?
Never work on a circuit until it has been identified, disconnected from every source of energy, secured against being switched back on, proved dead and earthed at the work point as the site procedure requires. Each step is done in order and none is skipped.
Q2Why is isolation so important on AGL circuits?
A series lighting loop is fed by a CCR that can produce a high voltage. The loop current, commonly up to 6.6 A, and the voltage can both be lethal, so the circuit must be made safe before any cable, connector or transformer is touched.
Q3Is switching off at the CCR enough?
No. Switching off is only the first part. The supply must be isolated and locked off, and the circuit must be tested to prove it is dead. Never assume a circuit is dead because a switch is off.
Q4What does 'prove dead' mean?
Testing with an approved voltage indicator, which is itself checked on a known source before and after, to confirm there is no voltage at the point of work.
Q5Why earth the circuit?
Earthing at the work point gives any stored charge, induced voltage or accidental back-feed a safe path, so it cannot reach the person working.
Q6Who can remove the lock and tag?
Only the person who applied them, or someone authorised under the site procedure. Locks and tags are not removed to save time.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.